battery based regulated power supply

[preetikarki]

i want to make a battery based regulated power supply, which gives 3 dc output voltages 5v/1 amp, 12v/2 amp and 24v/2 amp.i want to know that how can i calculate the value of batteries at input, as i want this supply to work for 1 hour at full load.

i guess 5v/1 amp=5watt
12v/2amp=24watt
24v/2amp=48watt. total=77 watt
if i use 24v, 7 amp(two 12v/7 amp battery in series)battery at input,then
77/24=3.2 amp
7amp hour/3.2 amp=2.18 hour
???? is this the right method or please make me right.

 

[alexan_e]

it depends if the regulators are pulse or linear.
a linear regulator will take the input voltage and reduce the output voltage by incresing the voltage drop across the regulator,
for example with input 24v and output 12v/2A the regulator will "consume" the 12v and will output the other 12v so the consumption will be 12*2 (as heat) + 12*2 (output) =48W.

If it is a switching regulator then it uses pulses to output the 24v with a lower duty ratio (50% for 12v) so that the mean value of the output is 12v after the output filter (coil,capacitor and diode),
in this case the consumption is still 48W but only for half of the total working period, for example 0.5sec on and 0.5sec off (in reality this would work in a frequency about 50K or 100KHz), this will give a mean consumption of 24W.

If you want an 24V output then 24v input will not be enough, it has to be a little higher than the output (unless you are using step up regulator which complicates things even more).

Alex

 

[Tahmid]

Hi,
It is the right method, given that you are using switching regulators(and of course assuming efficiency = 100% which it is not). Alex has already explained, let me explain a little.

Using switching regulator, with a 24v input and 12v 2A output, you draw 24W power as [output voltage * output current]. But with linear, you draw [output voltage * output current + dropped voltage * output current], ie. [output current * input voltage], so you draw 48W.

You also have to take into consideration that the conversion isn't 100% efficient for switching regulators as some is dissipated as heat and current is also required for the regulator to function. Thus you will get a backup time of less than what you have calculated.

As Alex mentioned, for 24v output, 24v input is not enough if you use simple regulators. Using a buck-boost regulator, however, it IS possible. But this also introduces more losses.

Switching can be done at frequencies in the range of 20-30kHz as well, ie, below 50kHz, but this requires larger inductors and capacitors.

So, for 5v and 12v, use step down regulators, the L49xx series of regulators from ST are a good choice. For 24v, use a buck-boost regulator. Or, you could increase the voltage input.

Hope this helps.
Tahmid.

 

[alexan_e]

As Alex mentioned, for 24v output, 24v input is not enough if you use simple regulators. Using a buck-boost regulator, however, it IS possible. But this also introduces more losses.

I didn't say it wasn't possible, i wrote that it can be done with a step up regulator.

If you want an 24V output then 24v input will not be enough, it has to be a little higher than the output (unless you are using step up regulator which complicates things even more).

Alex

 

[KJ6EAD]

You also need to carefully consider the realistic capability of the batteries in your application as opposed to the advertised rating. Capacity and discharge information such as this are available from the manufacturers: **broken link removed**.